Cleaning scanned magazine pages

[MrNiceButDim]

I'm totally inexperienced with GIMP but I chanced upon this recipe that would meet my needs for a rather big project I'm currently doing. It's about cleaning scanned magazine pages where the print on the back side shines through the thin paper making the reading less enjoyable.

GIMP Recipe:

1. Open a file.
   
2. Convert your document to grayscale: Image - Mode - Grayscale.
   
3. Select the background color: Select - By Color, click with mouse pointer on the color of the background.
   
4. Invert the selected color: Select - Invert.
   
5. Copy the selection: Edit - Copy.
   
6. Create a new file: File - New.
   
7. In the dialog of a new file, in field: Advanced Options choose: Fill with: White, hit Ok.
   
8. Click anywhere in the window of the new opened document, just to choose it.
   
9. Paste the content of a clipboard: Edit - Paste.
   
10. Add a new layer to enhance the black text: Layer - New from Visible, in the layer's palette, in field: Mode: choose Multiply.
    
11. Combine two layers: Layer - Merge Down.
    
12. Save the result as a JPEG file: File - Export As, choose jpeg and set the quality at least 60.
    
    

I have a couple of questions.

•  When getting to "Combine two layers: Layer - Merge Down." the Merge Down option is grayed out so I can't apply that step.
  

What could be the reason? And why does it say "Combine two layers" when there is three?

1.Floated selection (Pasted Layer)
2.Visible
3.Background

•  When selecting a background color to be - erased/turned to white - what would be the best way to pick a wider range of grayness than the rather sparse instruction the recipe suggests "By Color, click with mouse pointer on the color of the background."